The present invention relates to a method of applying coatings which contain only small amounts of gaseous impurities, in particular oxygen.
The application of refractory metal coatings to surfaces exhibits numerous problems.
In conventional processes, the metal is completely or partially melted in most cases, as a result of which the metals readily oxidise or absorb other gaseous impurities. For this reason, conventional processes such as deposition-welding and plasma spraying must be carried out under a protecting gas or in vacuo.
In such cases, the outlay in terms of apparatus is high, the size of the components is limited, and the content of gaseous impurities is still unsatisfactory.
The pronounced introduction of heat transmitted into the object to be coated leads to a very high potential for distortion and means that these processes cannot be employed in the case of complex components, which often also contain constituents that melt at low temperatures. Complex components must therefore be taken apart before they are re-processed, with the result, in general, that re-processing is scarcely economical and only recycling of the material of the components (scrapping) is carried out.
Moreover, in the case of vacuum plasma spraying, tungsten and copper impurities, which originate from the electrodes used, are introduced into the coating, which is generally undesirable. In the case of, for example, the use of tantalum or niobium coatings for corrosion protection, such impurities reduce the protective effect of the coating by the formation of so-called micro-galvanic cells.
Moreover, such processes are processes of melt metallurgy, which always involve the inherent disadvantages thereof, such as, for example, unidirectional grain growth. This occurs in particular in laser processes, where a suitable powder is applied to the surface and melted by means of a laser beam. A further problem is the porosity, which can be observed in particular when a metal powder is first applied and is subsequently melted by means of a heat source. Attempts have been made in WO 02/064287 to solve these problems by merely melting on the powder particles by means of an energy beam, such as, for example, laser beams, and sintering them. However, the results are not always satisfactory and a high outlay in terms of apparatus is required, and the problems associated with the introduction of a reduced but nevertheless high amount of energy into a complex component remain.
WO-A-03/106,051 discloses a method and an apparatus for low pressure cold spraying. In this process a coating of powder particles is sprayed in a gas substantially at ambient temperatures onto a workpiece. The process is conducted in a low ambient pressure environment which is less than atmospheric pressure to accelerate the sprayed powder particles. With this process a coating of a powder is formed on a workpiece.
EP-A-1,382,720 discloses another method and apparatus for low pressure cold spraying. In this process the target to be coated and the cold spray gun are located within a vacuum chamber at pressures below 80 kPa. With this process a workpiece is coated with a powder.